Biochemical treatment of polluted water to remove soluble organic material, nitrogen, phosphorus and the like has become of particular interest, and particularly with treatment utilizing the action of microorganisms. One aspect in particular is the biofilm process where microorganisms are adhered to the surface of a solid and the microorganisms decompose pollutants such as nitrogen in the water.
Of particular interest in the biofilm process is a fluidized bed process wherein a large, effective surface area of the biofilm is required.
In most processes involving the culture of microorganisms, there is a desire to provide as much effective surface area on which microorganisms can grow per unit volume of culture medium as possible, with the confines of an overall cost effective filter system.
Although this invention in its broadest scope is not limited to aquaculture systems, applicant sets out its invention, both for the background of the invention and preferred embodiments, exemplifying aquaculture systems.
Aquaculture, the culture of organisms such as fish in water, requires good quality water in large quantities. In view of the diminishing availability of appropriate natural sites, increasingly stringent effluent control regulations, and the need for cost effective controlled environments to optimize year round growth, water re-use technology is becoming increasingly recognized as essential to aquaculture systems.
In all water re-use systems, it is essential that solid wastes and dissolved wastes be removed and that oxygen utilized by the fish and bacterial growth be replenished.
Nitrifying bacteria cultured in filter beds are traditionally used in re-use systems to effectively remove potentially toxic ammonia by converting it to nitrate. The bacteria require well oxygenated water and a surface to grow on in addition to the ammonia in the water. Bacteria filters normally consist simply of inert material such as stone or gravel or plastic media which provide high surface area per unit volume for bacterial growth.
The key to high efficiency biofilter design is to incorporate as much effective surface area per unit volume as possible. To date, most biofilters have been quite bulky and relatively inefficient. The technologies primarily used to date have been based on municipal water purification systems. For example, a bed of gravel, which has a large particle surface area, is installed in a tank and waste-water is flushed through the bed. Bacteria will grow on the surfaces of the gravel particles but after a time a combination of the growing bacterial film and unfiltered solid waste particulates clogs the pores between the gravel particles. Short circuiting of the flow of water--that is, channelling of the flow, takes place and a large volume of the bed becomes inactive. Also, anaerobic conditions develop, that is, conditions of low oxygen and the generation of potentially toxic wastes. Accordingly, these types of systems require substantial maintenance including stirring and backwashing in order to clean the gravel filters on a regular basis.
In an attempt to remedy the above problem in commercial sewage treatment systems, moulded plastic shapes of various configurations, relatively large compared to the size of gravel particles, are installed in bulk in large tanks. The plastic shapes are designed with a lot of voids or spaces in them thereby creating a static bed with channels large enough that even with the accumulating growth of a biological film, there is not the tendency for clogging as in a gravel bed. The plastic media are a compromise between the need to maximize surface area per unit volume and the requirement for a relatively low maintenance system. Nevertheless, with this system, problems eventually occur with short circuiting and the development of anaerobic areas necessitating periodic flushing and cleansing maintenance of the filter.
Recently, there is increasing interest in fluidized bed biofilters wherein very fine particles, providing a large surface area for the growth of bacteria, are kept in motion so that there is not a gradual build up of a large amount of biofilm on the particles. Because the particles are in motion, there is a tendency for them to self-scour, thus providing an optimal thin, active aerobic biofilm on each particle. In theory, this type of system is ideal because every potential (particle) surface in the filter is covered with a good quality bacterial growth and the scouring action abrades any excess biofilm growth off the surfaces. High density sand particles, kept in motion and suspension by high pressure water jets, have been used effectively in these systems. However, such systems take a substantial amount of energy to operate.
Further, to date technologies used for water re-use in aquaculture systems have primarily been centralized filter designs where one large unit is custom engineered to serve an entire facility. Generally the designs have been similar to those used for municipal sewage treatment and they are not appropriate for commercial . aquaculture systems. More particularly, in centralized systems, all water from each fish culture tank must be pumped to a central filter and returned to each individual tank. This requires a substantial amount of energy for pumping and expensive pipes, valves and controls. Further, because the water from all tanks is combined for processing, there is a rish of spreading disease from one infected tank to all other tanks. Moreover, hatcheries are generally designed for a given production rate and the central filter plant is scaled accordingly. It is therefore difficult to expand without considerable expense in replicating the central treatment or filter plant. Central treatment plants are not off the shelf technology and therefore involve costly engineering. Further, because solid wastes from each fish tank are transported to the central plant, they are broken down and stirred into the cultured water as they travel at relatively high velocity in the pipes. Provision for settling out or filtering out the wastes must be provided. Because the solid wastes are held in the circuit of the filtration system for some time, a considerable amount of secondary decomposition of the solid wastes occurs generating dissolved wastes which in turn must be filtered out. Ideally solid wastes are removed instantaneously so they do not decompose. This is difficult with a centralized design.
Accordingly, with respect to the above description pertaining to aquaculture, it will be appreciated that there is a desire to provide a system which is of the fluidized bed type but which is much less energy demanding and a system which is susceptible to a modular design enabling each tank to be outfitted with a low cost biofilter system, whether such tank is part of a new system or whether it is part of an existing system being retrofitted.
However, it should not, as previously noted, be assumed that applicant's invention is limited in its broader aspects solely to aquaculture systems. Aspects of the invention pertain to the provision of pellets for use in a fluidized bed type of system to provide optimum surface configuration and maximum surface area per unit volume for the culture of microscopic organisms. Applicant's pellets and the fluidized system are adaptable for use in non-aquaculture systems such as in other water purification systems and in chemical, biological and fermentation systems. Applicant's invention is directed to providing an environment for promoting the growth of microorganisms, i.e. providing maximum effective surface area per unit volume for microorganism growth on an ongoing basis. Accordingly aspects of the invention should be adaptable for example to the anaerobic production of methane gas, using methane as a fluidizing gas or in a fermentation process for the production of alcohols using carbon dioxode as the fluidizing gas.
It is also to be noted that in those prior art systems that have used fluidized particles wherein the particles of a density greater than 1.1 and granular in size, it is difficult to have a cost effective system with favourable height to width ratio, such systems requiring a significant height in order to maintain the granular particles within the fluidized bed system. Otherwise systems for separating the granular particles which flow out of the fluidized bed system with the outlet flow or effluent is required.
Accordingly, although some fluidized bed systems may use smaller particles, such systems are not cost-effectively usable, because of unfavourable height-to-width ratios in a system virtually within tanks of the water to be treated such as aquaculture tanks.